Vortex fluid meters or flowmeters are widely known and usually comprise a pipe along which there flows a fluid whose volume and/or flowrate are to be measured. An obstacle is placed in the fluid flow inside the pipe so that when the fluid flow meets said obstacle fluid vortices are generated and separate from the obstacle in an oscillatory manner. This kind of meter also includes means for determining the volume of fluid from these oscillations. These means are usually on the obstacle. The principle of measuring the fluid volume in this type of meter is based on the fact that the frequency of oscillation of the vortices is approximately proportional to the speed of the fluid in the pipe and the aforementioned means detect a signal corresponding to the oscillations of said vortices. This signal can be a differential pressure, for example.
The purpose of a vortex fluid meter is to provide an accurate and reliable measurement of the flowrate or the volume of the fluid flowing through the pipe for wide ranges of Reynolds numbers. To this end the Strouhal number (the ratio of the product of the oscillation frequency multiplied by the obstacle diameter to the speed of the fluid) must be constant for varying Reynolds numbers (the ratio of the product of the fluid speed multiplied by the pipe diameter to the dynamic viscosity of the fluid). Recent research in this field has lead to the design of vortex fluid meters with optimized obstacle shapes and dimensions and that are totally satisfactory for high Reynolds numbers, for example Reynolds numbers in the order of 260,000.
Patent application EP-A-0 408 355 describes a vortex flowmeter including a pipe in which the fluid flows, a double obstacle disposed in the flow of fluid and means for measuring the frequency of separation of the vortices generated by said double obstacle.
The double obstacle comprises an elongate first or upstream obstacle whose upstream face is convex and whose downstream face is plane, and a second or downstream obstacle remote from the first and whose upstream face is plane and whose downstream face is concave or plane. In the case of a gas, flowmeters of this kind are not suitable for measuring Reynolds numbers less than 146,000 and in particular less than 33,000. At high Reynolds numbers the shear layers that develop at the lateral edges of the obstacle separate and form fluid vortices whose flow is turbulent and the corresponding Strouhal number is relatively constant. At low Reynolds numbers, however, the shear layers become progressively laminar, the vortices form more quickly, the oscillation frequency of the vortices falls more slowly than the speed of the fluid, and the Strouhal number rises quickly, introducing error into the fluid volume measurement.